Whether space ever ends is a hard question. There is a limit to the space that we can see, because if there is stuff beyond 15 - 20 billion light years (the age of the Universe) the light from there hasn't reached us yet. So we don't know.

The reason it is dark in space actually has to do with
the fact that the Universe we can see is finite (has limits),
either finite in size or age. They are essentially the same thing
because the finite age of the Universe (15 - 20 Billion years) means
that light from stars furthur away than 15 to 20 Billion light years
hasn't reached us yet. So the Universe looks to be 15 to 20
Billion light years in radius, even if it's
bigger.

The fact that the sky is dark is known as
Olber's paradox. If the Universe was infinite, there would be a star
in every direction, and the sky would be uniformly bright. Instead
the stars and light are spread out enough that it is
dark.

What if the energy driving and directing the Big Bang was God? The Big Bang theory does not seem to account for the source of any energy/matter. What if it derived from a God in the process of creating a Universe and His method was the Big Bang?

Your question has come to NASA, and I'm sure you know
that we answer scientific questions, not religious ones. But this
question has been addressed by our sister site, Imagine
the Universe!.

Does the "Big Bang" mean that the Universe started out as an extremely large supernova-like event?

Other than the fact that a supernova and the Big Bang
are both "explosions", there is little similarity. Cosmic
Mystery Tour at UIUC gives a definition of the Big Bang, and Timeline
of the Universe at NASA JPL talks about what scientists believe
happened in the time immediately afterward.

I heard that scientists are working on a "Grand Unified Theory" that would explain everything. However, isn't the Big Bang theory pretty much a "theory of everything?"

No, the Big Bang is just a theory on the origin of the universe. The Grand Unified Theory attempts to combine the four forces that are known (gravity, electromagnetism, strong nuclear, and weak nuclear) into one theory. As yet, scientists haven't been able to do this.

Does electricity play an organizing role in space?
Astronomers see magnetism everywhere but rarely have I seen mention of
the electric currents that must be present to power them.

Magnetic fields are due to currents. The currents can occur over
vast scales, intermediate scales very easy for a human to grasp, or
over very minute scales. Currents are just moving charge, and if there
is a net motion of charge, there is a current.

Gravitational force seems to give structure to the universe and
bind together what is bound on the largest of scales. There are
several reasons why gravity dominates and not electric or magnetic
fields.

The first is that magnetic forces vary as the inverse of distance
cubed, while the others vary as the inverse of distance squared, so
magnetic forces are the shortest range of the three.

Another thing to consider is that electric forces can both attract and
repel. A positive charge tends to become surrounded by negative
charges. From a distance, they neutralize one another and no net
charge is seen. In plasmas this is called Debye shielding, and over
distances bigger than a surprisingly small distance (Debye length),
one does not observe the build-up of charge. This tends to cancel
electric forces at these distances.

Nothing neutralizes gravity. So gravity is left to have its way over
the vast distances of space.

So, if magnetic fields exist in space, where are the currents? There
is more subtlety to this question than you may appreciate. The first
point is that there are currents -- there have to be currents to get
magnetic fields. However, consider this: a current confined to a wire
produces a magnetic field that fills the space around the wire. So we
can measure magnetic fields everywhere, but the currents that produce
them may be much more isolated.

Let's think about what creates a current. Two things cause currents to
flow: separation of charge and electromotive force via Faraday's Law.
The latter can be used to create separation of charge, as when you
charge a battery. Separation of charge in space can be accomplished,
but it is difficult. Many of the currents that flow in space are the
result of an electromotive force. When you consider that a conducting
fluid, such as plasma, will convect the magnetic field and carry it
with the flow, it isn't hard to imagine that the magnetic field can be
rapidly changing at any point in space and can become quite complex.
So are the currents that support the fields.

In magnetospheric physics, there are two schools. One school attempts
to understand the complex dynamics of the magnetosphere by specifying
the currents and the other by way of the magnetic fields. If the
currents are localized, we may not have spacecraft in the proper place
to measure them. Since the magnetic fields fill space, it is more
likely that we can study them, in an attempt to better understand the
currents that produce them. Add to that the difficulties in measuring
currents in space, and you may begin to appreciate why so many people
choose to address the magnetic fields and not the currents that
produce them. However, the current approach has a strong following in
magnetospheric physics.

Let me recommend a book to you: Eugene Parker's "Conversations on
Electric and Magnetic Fields in the Cosmos". It is very good, and he
tackles exactly the problem you pose. Gene is in many ways the father
of modern space physics, and while he is now retired, he speaks and
writes wonderfully. The book is excellent, but it is advanced.

I've offered you my prejudicial view of why people talk more about
magnetic fields than the currents that produce them. One thing is
true -- you can't have one without the other. You need to decide which
half of the pair will give you the information you need, and then
decide how to get at it. It's a complicated
subject.

Is there a difference between cosmic rays and cosmic background radiation?

Cosmic rays are particles - the nuclei of elements in the periodic table. They have nothing to do with the cosmic microwave background. The cosmic background radiation is the thermal radiation left over from the Big Bang - it consists of photons only - not particles. The only thing they have in common is the word "cosmic". Please visit the COBE home page for more on the cosmic background.

What is the approximate temperature of the Universe, and how can it be calculated?

The entire Universe is filled with the remnants of the Big Bang, in the form of photons (electromagnetic packets). They have cooled down to about 2.7 Kelvin or 2.7 degrees above absolute zero (-270.7 degrees Centigrade). So this is the temperature of space. It can be calculated from the expansion of the Universe, and it has been measured.

I'm in the fourth grade. Why is space so cold if there are
so many stars?

That is a question that scientists thought about
hundreds of years ago. The answer is that space is cold for the same
reason the night sky is dark.

Stars give off both heat and light. The night sky is dark
because, although there are billions of stars, there are many more
directions in the sky that don't point to stars than do point
to stars. So you get a dark sky because the sky is mostly dark with
only many small points of light.

The dark parts of the sky are also cold (2.7 degrees above
absolute zero). So in space you get a sky that is mostly cold with
only many small points of heat. That still adds up to very
cold.

What are gravity waves? I understand that someone won the Nobel Prize for discovering gravity waves. Gravity can be illustrated by taking a heavy object and placing it on an elastic fabric; the more the fabric curves, the stronger the gravity. But where do the waves come in?

Gravity waves are ripples on the elastic fabric of space. If you hit or ring that heavy object, a ripple will move outward. This is a gravity wave, and it travels at the speed of light. Hulse and Taylor won the Nobel prize in 1993 for discovering a binary pulsar whose period was slowing down exactly as predicted if the pair was losing energy by giving off gravity waves.